encoding/json: document Encoder will terminate each JSON value with '\n'

Fixes #7767.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/87420043

src/pkg/encoding/json/stream.go

@@ -148,7 +148,8 @@ func NewEncoder(w io.Writer) *Encoder {
 	return &Encoder{w: w}
 }
 
-// Encode writes the JSON encoding of v to the stream.
+// Encode writes the JSON encoding of v to the stream,
+// followed by a newline character.
 //
 // See the documentation for Marshal for details about the
 // conversion of Go values to JSON.